For example, a single crystal of a metal fluoride such as calcium fluoride, barium fluoride, magnesium fluoride or lithium fluoride has a high transmittance over a wavelength band within a wide range, and has a low dispersion and an excellent chemical stability.
For this reason, there is a high demand for single crystals of these metal fluorides for use as optical materials in various devices using a laser having an ultraviolet wavelength or a vacuum ultraviolet wavelength, in a lens of a camera or a CVD device and as a window material.
In the single crystals of the metal fluorides, particularly, the single crystal of calcium fluoride has been expected as a projection lens to be used for an F2 excimer laser (wavelength: 157 nm) in a vacuum ultraviolet wavelength region which has been developed as a short-wavelength light source in the next generation in an optical lithographic technique to be a main stream in the production of a semiconductor device.
Conventionally, it is known that such a single crystal of a metal fluoride can be manufactured by a crucible dropping method (Bridgman-Stockbarger method) and by a pulling method (Czochralski method).
The pulling method is not restrained by a crucible during the raising of a single crystal. Therefore, a distortion is hardly generated on the crystal, and furthermore, an impurity can be reduced by a segregation phenomenon during the raising. Consequently, the pulling method has been generally used in the manufacture of a semiconductor single crystal such as silicon or germanium.
In the pulling method, a seed crystal comprising a single crystal to be an object is caused to come in contact with a molten solution of a single crystal manufacturing material in a crucible, and subsequently, the seed is gradually pulled out of the heating area of the crucible and is thus cooled, thereby raising the single crystal below the seed crystal.
In order to execute such a pulling method, conventionally, there has been used a single crystal pulling apparatus 100 shown in FIG. 2.
The single crystal pulling apparatus 100 comprises a chamber 102 constituting a crystal growth furnace, and the chamber 102 includes a rotatable support shaft 106 to penetrate through a bottom wall 104 of the chamber 102.
The lower end of the support shaft 106 penetrates through the bottom wall 104 of the chamber 102 and is extended out of the chamber 102, and comes in contact with a cooler and is then connected to a driving mechanism for rotating and vertically moving a crucible 110, which is not shown.
Moreover, a base 108 is fixed to the support shaft 106 and the crucible 110 is mounted on the upper surface of the base 108. A molten solution 112 of a single crystal manufacturing material is filled in the crucible 110.
A melting heater 114 is erected from the bottom wall 104 of the chamber 102 to surround the crucible 110. Furthermore, a heat insulating wall 116 is erected from the bottom wall 104 of the chamber 102 to surround the melting heater 114 and the crucible 110.
Usually, the height of the upper end of the melting heater 114 is almost equal to that of the upper end of the crucible 110.
On the other hand, a vertical movable and rotatable single crystal pulling bar 122 is hung down from an upper wall 118 of the chamber 102 through an opening portion 120 by means of the driving means which is not shown. A seed crystal 124 is attached to the tip of the single crystal pulling bar 122 through a holding tool 123, and the seed crystal 124 is provided to be positioned on the central axis of the crucible 110.
In the single crystal pulling apparatus 100 having such a structure, the single crystal pulling bar 122 is brought down toward the molten solution 112 of the single crystal manufacturing material set in a melting state in the crucible 110 by the heating operation of the melting heater 114. Then, the lower end plane of the seed crystal 124 provided on the tip of the single crystal pulling bar 122 comes in contact with the molten material solution 112 in the crucible 110 and the single crystal pulling bar 122 is thereafter pulled up so that a single crystal 126 is grown under the seed crystal 124.
In FIG. 2, the reference numeral 128 denotes an inspection window provided on the top of the chamber.
In the conventional single crystal pulling apparatus 100 having such a structure, for example, which has generally been used in the manufacture of a single crystal having a comparatively high crystal growth speed of silicon or the like, the upper end of the heat insulating wall 116 surrounding the melting heater 114 and the crucible 110 is generally provided to have a slightly greater height than the height of the upper end of the crucible 110 as shown in FIG. 2.
More specifically, since the crystal growth speed is comparatively high in the single crystal of silicon or the like, the heat of the crucible 110 can be sufficiently retained. In order to radiate a crystallization heat, it is preferable that the upper end of the heat insulating wall 116 should be positioned to have such a height as to be slightly greater than the height of the upper end of the crucible 110.
In the case in which a single crystal of a metal fluoride is manufactured by using such a conventional single crystal pulling apparatus 100, however, there is often a problem in that a crack is generated on the single crystal thus pulled up.
The reason is that the crystal growth speed of the single crystal of the metal fluoride is extremely lower than that of the single crystal of silicon or the like. More specifically, in the case in which the pulling apparatus 100 including the upper end of the heat insulating wall 116 positioned to have such a height as to be slightly greater than the height of the upper end of the crucible 110 is used for the single crystal of the metal fluoride having an extremely low crystal growth speed as described above, the heat insulating wall 116 is not present in the single crystal pulling region provided above the crucible 110. As a result, the gradient of a reduction in a temperature is increased after all and it is hard to grow a crystal stably and slowly.
For this reason, a single crystal pulling apparatus disclosed in Japanese Laid-Open Patent Publication No. Sho 63(1988)-270385 has been proposed for an apparatus for pulling a single crystal of an oxide such as LiTaO3 having a comparatively low crystal growth speed (see pages 1 to 2 and FIG. 2 in the publication).
A single crystal pulling apparatus 200 disclosed in the Japanese Laid-Open Patent. Publication No. Sho 63(1988)-270385 has a structure shown in FIG. 3.
More specifically, in the single crystal pulling apparatus 200, a base 204 is provided on a furnace body bottom portion 202 and an alumina table 206 is provided on the base 204. A crucible 210 formed of iridium is provided above the alumina base 206 through a crucible base 208 and an after heater 212 formed of iridium is provided on the crucible 210.
A temperature retaining cylinder 214 is provided to surround the circumference of the crucible 210 and a single crystal pulling region, formed above the crucible 210. Moreover, a zirconia bubble 216 is provided between the temperature retaining cylinder 214 and the crucible 210. A high-frequency coil 218 for heating is provided to surround the temperature retaining cylinder 214.
Furthermore, an upper cover (a ceiling board) 222 is provided on an opening portion 220 formed on the upper end of the temperature retaining cylinder 214, thereby closing the opening portion 220. The ceiling board 222 is provided with an inserting hole 225 for a single crystal pulling bar 224 and the single crystal pulling bar 224 is inserted through the inserting hole 225. Moreover, a seed crystal (seed) 226 is provided on the tip of the single crystal pulling bar 224. When the single crystal pulling bar 224 is pulled up, a single crystal 228 is grown under the seed crystal 226.
In the Japanese Laid-Open Patent Publication No. Sho. 63(1988)-270385, in the single crystal pulling apparatus 200 having such a structure, it has been proposed that an opening area between the ceiling board 222 and the single crystal pulling bar 224, that is, an opening area between the single crystal pulling bar 224 and the inserting hole 225 is regulated to control a temperature gradient in a position placed just above a molten material solution by 5 mm in the crucible 210, thereby preventing the generation of a crack in a single crystal.
According to the single crystal pulling apparatus 200 having such a structure, the single crystal pulling region is held in a chamber (a single crystal pulling chamber) 230 formed by the temperature retaining cylinder 214 and the ceiling board 222. Consequently, a heat retaining property can be greatly enhanced and a gradient of a reduction in a temperature in such a direction as to go toward the upper part of the same region can be decreased considerably.
According to such a conventional single crystal pulling apparatus 200, therefore, it is possible to manufacture a single crystal of an oxide such as LiTaO3 with the generation of a crack suppressed considerably.
In the case in which a single crystal of a metal fluoride is manufactured by using such a single crystal pulling apparatus 200, however, the crystal growth speed of the single crystal of the metal fluoride is extremely low. As a result, with respect to the single crystal of the metal fluoride, the heat retaining property in the single crystal pulling region becomes excessive and the gradient of the reduction in the temperature in the single crystal pulling region becomes insufficient so that it is hard to sufficiently grow a single crystal in many cases.
On the condition that the single crystal is grown, similarly, the generation of the crack cannot be suppressed at such a high level as to be satisfied. In the case in which the metal fluoride is calcium fluoride or the case in which the crucible to be provided is a large-sized apparatus having an inside diameter of 11 cm or more, particularly, the crack is still generated considerably.
In the Japanese Laid-Open Patent Publication No. Sho 63(1988)-270385, furthermore, the material of the ceiling board 222 is not considered at all. From the fact that the thickness of the ceiling board 222 is greater than that of the heat insulating material of a side wall and the fact that the hatching pattern of the thickness of the ceiling board 222 is displayed to be the same as the hatching of the alumina table 206 to be a bottom cover, it can be guessed that the ceiling board 222 is formed by a heat insulating material as the same member as the alumina table 206 or the temperature retaining cylinder 214.
According to the investigations of the present inventors, therefore, the metal fluoride such as calcium fluoride is particularly required to be cooled uniformly and gradually in order to grow a stable crystal. On the other hand, in a method of controlling a reduction in the temperature of the single crystal pulling region by regulating the opening area between the single crystal pulling bar 224 and the inserting hole 225 as in the Japanese Laid-Open Patent Publication No. Sho 63(1988)-270385 and a method in which the heat insulating properties and coefficient of thermal conductivity of the ceiling board 222 and the temperature retaining cylinder 214 are not considered at all, it can be guessed that non-uniformity is generated in a temperature distribution in a radial direction or a vertical direction, resulting in a hindrance to a stable crystal growth.
Furthermore, Japanese Laid-Open Patent Publication No. Hei 11(1999)-21197 has disclosed a single crystal pulling apparatus 300 for the metal fluoride such as calcium fluoride.
In a single crystal pulling apparatus 300 disclosed in the Japanese Laid-Open Patent Publication No. Hei 11(1999)-21197, a crucible 304 is provided in a growth furnace chamber 302 and a heater 306 is provided around the crucible 304 as shown in FIG. 4.
A heat insulating member 308 is provided to surround the crucible 304 and the heater 306. An inward extended portion 310 is provided in the upper part of the heat insulating member 308 to cover the upper portion of the heater 306.
Furthermore, a molten solution 316 is filled in the crucible 304 and a seed crystal 314 provided on the tip of a single crystal pulling bar 312 is constituted to come in contact with the molten solution 316 in the crucible 304.
In the single crystal pulling apparatus 300, however, the inward extended portion 310 of the heat insulating member 308 simply covers the upper part of the heater 306 in order to efficiently carry out the heating operation of the heater 306. Furthermore, an opening having considerable large area is present between the crucible 304 and the single crystal pulling bar 312.
In the single crystal pulling apparatus 300, accordingly, the upper end of the heat insulating member 308 is basically positioned to have a slightly greater height than the height of the upper end of the crucible 304 in the same manner as in the conventional single crystal pulling apparatus 100 shown in FIG. 1. In the case in which the single crystal pulling apparatus 300 is used for the single crystal of the metal fluoride which has an extremely low crystal growth speed, a gradient of a reduction in a temperature is increased after all, because the heat insulating member 308 is not present in the single crystal pulling region provided above the crucible 304. As a result, it is difficult to grow a crystal stably and slowly. Therefore, a crack is generated on the single crystal which is pulled up.
Also in all the conventional single crystal manufacturing apparatuses, therefore, it is a great object to develop a single crystal pulling apparatus which can improve the nonuniformity of a temperature distribution in the single crystal pulling region and can manufacture the single crystal of a metal fluoride well without generating a crack.
In order to solve the problems, the present inventors vigorously made a study. As a result, they found that the above-mentioned problems can be solved by forming a ceiling board with a material having a high coefficient of thermal conductivity in a single crystal pulling apparatus, and completed the present invention.